Electrochromic materials and devices have been developed as an alternative to passive coating materials for light and heat management. Such materials and devices have been used in architectural windows, including insulated or active glazing units, and in automobile windows, including windshields and rear-view mirrors. In contrast to passive coating materials, electrochromic devices employ materials capable of reversibly altering their optical properties following electrochemical oxidation and reduction in response to an applied potential. The optical modulation is the result of the simultaneous insertion and extraction of electrons and charge-compensating ions in the electrochemical material lattice.
In general, electrochromic devices have a composite structure through which the transmittance of light can be modulated. FIGS. 1A and 1B illustrate plan and cross-sectional views, respectively, of a typical prior art electrochromic device 20. The device 20 includes isolated transparent conductive layer regions 26A and 26B that have been formed on a substrate 34, such as glass or optical glazing. In addition, the device 20 includes a counter electrode layer 28, an ion conductive layer 32, an electrochromic layer 30, and a transparent conductive layer 24, which have been deposited in sequence over the conductive layer regions 26. The counter electrode layer, the ion conductive layer, the electrochromic layer, and the two transparent conductive layers are collectively referred to as an “electrochromic stack.” It is to be understood that the relative positions of the electrochromic and counter electrode layers of the device 20 may be interchanged. Further, the device 20 includes a busbar 40 which is in contact only with the conductive layer region 26A, and a busbar 42 which may be formed on the conductive layer region 26B and is in contact with the conductive layer 24. The conductive layer region 26A is physically isolated from the conductive layer region 26B and the busbar 42, and the conductive layer 24 is physically isolated from the busbar 40. Although an electrochromic device may have a variety of shapes, such as one including curved sides, the illustrative, exemplary device 20 is a rectangular device with the busbars 40 and 42 extending parallel to each other, adjacent to respective opposing sides 25, 27 of the device 20, and separated from each other by a distance W. Further, the busbars 40 and 42 are connected by wires to positive and negative terminals, respectively, of a low voltage electrical source 22 (the wires and the source 22 together constituting an “external circuit”).
Referring to FIGS. 1A and 1B, when the source 22 is operated to apply an electrical potential across the busbars 40, 42, electrons providing a current flow from the busbar 42, across the transparent conductive layer 24, and into the electrochromic layer 30. In addition, if the ion conductive layer 32 is an imperfect electronic insulator as is the case in many thin film electrochromic devices, a small current, commonly referred to as a leakage current, flows from the busbar 42, through the conductive layer 24 and the electrochromic layer 30, and into the ion conductive layer 32. Further, ions flow from the counter electrode layer 28, through the ion conductive layer 32, and to the electrochromic layer 30, and a charge balance is maintained by electrons being extracted from the counter electrode layer 28, and then being inserted into the electrochromic layer 30 via the external circuit. As the current flows away from the busbar 42 across the conductive layer 24 and towards the busbar 40, voltage is dropped by virtue of the finite sheet resistance of the conductive layer 24, which is typically in a range between about 3 and about 20 Ohms/square. In addition, current flowing across the conductive layer 24 is incrementally reduced, as current is drawn through the combination of the layers 30, 32 and 28 (“intermediate stack”) to produce the electrochromic coloration in the device 20.
Applying the electrical potential across the busbars 40, 42 may be beneficial for preventing excess light or glare from being transmitted through the optical glazing. However, when the device 20 darkens, light (e.g., daylight) transmitted through the device is filtered according to the spectral properties of the device.
As illustrated in the graph shown in FIG. 2, the transmissivity of light through a tinted substrate, such as an electrochromic insulated glazing unit (“EC IGU”), varies based on the wavelength of the light. Accordingly, some wavelengths of light are absorbed or reflected more than others such that the color of light passing through electrochromic glass will be altered. As a result, the view through the optical glazing often takes on an unnatural or unconventional hue, which is often a bluish hue as light waves producing this hue have a higher transmissivity through glass than other light waves. For example, if a view through heavily tinted glass produces a deep bluish hue, such an appearance is due to the glass heavily absorbing green and red portions of the spectrum of light passing through.
Additionally, the light transmitted through the glazing also often takes on such an unnatural, often bluish hue. The perception of such an unnatural hue by viewers is caused by the color saturation, i.e., the intensity of the color of the object being viewed, and the color rendering index (CRI), which is a measure of the ability of a viewer to distinguish between colors under a given light source.
In some situations, some users of such a device, or a vehicle or architectural structure equipped with such a device, find the unnatural blue-tinted appearance of the environment, which may an interior or exterior, viewed through the device to be undesirable, and in some instances even unpleasant. Such users often prefer that the appearance of such an environment be “neutral” (e.g., having an appearance similar to that under natural or ambient light conditions, having an appearance that comports to conventional indoor or outdoor lighting standards, etc.). Also, in some situations, a user in an interior space, such as a vehicle or building interior, may find the tinted appearance of the interior space, as colored by the light passing through the glazing, to be undesirable, and in some instances unpleasant. For example, a tinted hue caused by light passing through electrochromic glass installed in an art museum can alter the look of art to guests within the museum significantly and be considered by the guests as undesirable or unacceptable. Such users or guests could prefer that the appearance of the interior space, such as the museum in the prior example, be “neutral.”
Thus, there is a need for a device that prevents excess light or glare from being transmitted through the optical glazing while maintaining the appearance of the environment at an acceptable or pleasing hue, and in some instances at a neutral hue.